In general, in a brake system for a vehicle, a hydraulic pressure that is a braking pressure is transferred to the wheel brakes by operating the brake pedal to decelerate or stop the vehicle. However, because the brake system is actuated to stop a traveling vehicle by force, which causes the vehicle to skid forward depending on the road condition or braking pressure.
A brake system that electronically controls the braking pressure for the wheel brakes is widely used in vehicles to prevent the slip, and, for example, an ABS (Anti-Lock Brake System), a TCS (Traction Control System) that prevents excessive slip of the driving wheels due to sudden start or sudden acceleration, or an ESP (Electronic Stability Program) system that safely maintains the direction of a vehicle travel intended by the driver under any driving conditions by minimizing the difference of the actual direction from the direction of a vehicle travel intended by the driver.
These electronic control brake systems are provided with a solenoid valve to control hydraulic pressure in the brake wheel cylinders. The solenoid valve was controlled by an on-off control method in the related art, but, recently, is controlled by pulse width modulation (referred to as PWM hereafter) to appropriately adapt to various conditions.
For example, as for an ABS/ESP, when a normal open-typed solenoid valve is operated, a control unit (commonly called ASB/ESP HU (Hydraulic Unit)) adjusts a preset duty ratio of PWM signals and actuates the normal open-typed solenoid valve to control the pressure of the brake wheel cylinders.
The solenoid valve that is installed in the oil pump housing block blocks the flow passage when working fluid flows from the master cylinder to the wheel cylinder and opens the flow passage when working fluid flows from the wheel cylinder to the master cylinder. Therefore, the channel formed in the pump housing block allows oil to flow through an involved pump and accumulator as well as the solenoid valve.
Accordingly, the solenoid valve is installed such that the inlet/outlet of the solenoid valve is aligned with the inlet/outlet of the channel formed in the pump housing block.
Further, because a linear pressure should be controlled in order to control PWM of the solenoid valve, it is preferable for oil to flow into the solenoid valve toward the front of the plunger.
In general, however, a channel that is formed in a pump housing block including a solenoid valve is formed such that oil flows into the solenoid valve from the side. Accordingly, since the oil flows into the solenoid valve from other directions and not from the front of the plunger, it is difficult to control the PWM as well as the linear pressure.
Further, in order to overcome the above problems, when a channel is formed in the pump housing block to guide oil to the front of the plunger in the solenoid valve, that is, when the inlet and the outlet of the channel are switched from each other in the pump housing block, the manufacturing cost increases. Furthermore, due to the change of the inlet port and outlet port of the channel, the entire size of the pump housing block increases, instead of reducing the size.